email sender is always from main domain...

I hsot severall domains on the server and now I noticed that for a particular domain (zice.ro) I only created 2 users, one is the admin the other one is a user. when the user sends an email I see username@mymaindomain instead of @zice.ro...

I am using squirrelmail for testing. I changed his emailaddress (sender info) to user@zice.ro inside squirrelmail but it keeps changing back...